Synthesis of benzo[b]thiophene 1,1-dioxides via Pd-catalyzed sulfinylation of aryl triflates and their use as large Stokes shift fluorophores for multicolor live-cell imaging with self-labeling tags

Benzo[b]thiophene 1,1-dioxide is a relatively underrepresented sulfone-embedded heterocycle with niche uses in material science (OLED emitters), pharmaceutical research (an electrophilic warhead in targeted covalent inhibitors) and as a structure unit in reversibly switchable diarylethene fluorophores. Benzo[b]thiophene 1,1-dioxides are most commonly prepared by oxidation of the corresponding benzothiophenes. Here, we propose an alternative approach based on Pd-catalyzed sulfinylation of ortho-carbonyl substituted aryl triflates followed by S-alkylation and Knoevenagel condensation. This methodology allows for an expedient access to diversely substituted benzo[b]thiophene 1,1-dioxides including 6-dialkylamino “push-pull” type fluorophores possessing large (>140 nm) Stokes shifts. Fluorescent labels derived from this core structure are compatible with live cell imaging using self-labeling protein tags (HaloTag, SNAP-tag and CLIP-tag). Together with the established live-cell fluorescent labels and orthogonal self-labeling tags, they allow for simultaneous observation of up to three intracellular targets below diffraction limit using fluorescence microscopy with three excitation lasers (485, 561 and 640 nm) and stimulated emission depletion (STED) at 775 nm.